[jdk21u-dev] RFR: 8317535: Shenandoah: Remove unused code

[William Kemper]

Removes unused and/or unreachable code.

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Commit messages:
 - Backport e942f368c370e059c654e33408940a987013a5c7

Changes: https://git.openjdk.org/jdk21u-dev/pull/194/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=194&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317535
  Stats: 185 lines in 20 files changed: 0 ins; 179 del; 6 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/194.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/194/head:pull/194

PR: https://git.openjdk.org/jdk21u-dev/pull/194